The disclosed embodiments relate generally to content addressable memory (CAM) cells.
A content addressable memory (CAM) device is a storage device that is particularly suitable for matching functions because it can be instructed to compare a specific pattern of comparand data with data stored in an associative CAM array. A CAM, also referred to as an associative memory, can include a number of data storage locations, each of which can be accessed by a corresponding address. Functionality of a CAM depends at least in part on whether the CAM includes binary or ternary CAM cells.
Typical binary CAM cells are able to store two states of information, a logic one state and a logic zero state. Binary CAM cells typically include a random access memory (RAM) cell and a compare circuit. The compare circuit compares the comparand data with data stored in the RAM cell and provides the match result to a match line. Columns of binary CAM cells may be globally masked by mask data stored in one or more global mask registers.
Ternary CAM cells are mask-per-bit CAM cells that effectively store three states of information, namely a logic one state, a logic zero state, and a don""t care state for compare operations. Ternary CAM cells typically include a second RAM cell that stores local mask data for the each ternary CAM cell. The local mask data masks the comparison result of the comparand data with the data stored in the first RAM cell such that, when the mask bit has a first predetermined value (a logic low, for example) its compare operation will be masked so that the comparison result does not affect the match line (e.g., always appears to match). The ternary CAM cell offers more flexibility to the user to determine on an entry-per-entry basis which bits in a word will be masked during a compare operation.
Many typical CAM devices used static memory technology. However, dynamic memory technology including dynamic random access memory (DRAM) devices is also being used because it can provide relatively denser and, therefore, larger memory arrays on the same size chip as similar arrays using static memory technology. The efficient search capabilities of CAM devices have proven useful in many applications including address filtering and lookups in routers and networking equipment, for example, and pattern recognition for encryption and/or decryption and compression and/or decompression applications, for example, as well as other pattern recognition applications. As the applications for CAM devices increase so to do the applications for denser DRAM-based CAM cells.